Experiments are currently in progress to gain insight into the neural basis of visual perception by examining the responses of foveal cortical cells in trained rhesus monkeys. Microelectrodes made of finely etched platinum iridium wire insulated with glass are lowered with the aid of an hydraulic micrometer drive system through the unopened dura into the occipital cortex of a monkey while he performs visual discriminations on a black-projected screen. Some experiments call for the monkey to fixate a small spot of light while the investigators map receptive field locations of single neurons in striate cortex and in extrastriate cortex buried in the lunate and inferior occipital sulci. On the basis of such recordings in a series of parallel electrode penetrations at 1 mm intervals over the foveal projection region, retinotopic maps are being constructed. Following electrolytic marking of selected penetrations the monkeys's brain is fixed, sectioned, stained, examined and the penetration sites reconstructed from tracings of serial brain sections. In other experiments, the monkey is trained to make forced choice visual discriminations (red vs. green, vertical vs. horizontal) while microelectrode recordings are obtained from single neurons whose responses may be involved in such discriminations.